1. Field of the Invention
The present invention relates to an edge rate control type output buffer circuit.
2. Description of the Related Art
An output buffer circuit has been described in a literature entitled “Design Guide For A Low Speed Buffer For The Universal Serial Bus” Revision 1.1 December 1996 by Intel Corporation as a conventional edge rate control type output buffer circuit. The conventional output buffer circuit is a CMOS output buffer circuit that is used as a buffer circuit of a USB (Universal Serial Bus).
This conventional output buffer circuit has a PMOS transistor and an NMOS transistor, connected to an output pad, respectively, as an output transistor. The PMOS transistor is controlled in a conductive state by an amplifier. The NMOS transistor is controlled in a conductive state by an amplifier that is different from the amplifier for controlling the PMOS transistor. These two amplifiers are controlled in operation respectively, in response to an enable signal. Inverted input terminals of these two amplifiers are connected to a bias circuit, respectively, and a given bias potential is supplied to these amplifiers. Non-inverted input terminals of these two amplifiers are commonly connected to each other. The common connection node is connected to a clamp circuit and a given potential is always supplied to the common connection node. A feedback capacitor element is connected between the common connection node and an output pad. The conventional output buffer circuit having the foregoing structure controls a throughput rate of an output waveform at the rising edge and the throughput rate of the output waveform at the falling edge (hereinafter referred to simply as rising edge rate and falling edge rate).
However, there is a case where the conventional output buffer circuit can not satisfy an AC standard of a USB, particularly, VCRS (crossover voltage) standard owing to change of operational environment such as variation of a power supply potential, change of an operational temperature or the like. There are following two reasons. One reason is that there occurs distortion in an output waveform because a given bias potential is always supplied to a common node. The second reason is that there occurs a distortion in an output waveform owing to influence of current which flows into through a pull-up resistor in the case where the conventional output buffer circuit is used as a low speed buffer of the USB.